1. Field of the Invention
The present invention relates generally to an electrochemical drive circuitry and method, such as may be employed in electroplating bath chemical monitoring.
2. Description of the Related Art
In the practice of copper interconnect technology in semiconductor manufacturing, electrolytic deposition is widely employed for forming interconnect structures on microelectronic substrates. The Damascene process, for example, uses physical vapor deposition to deposit a seed layer on a barrier layer, followed by electrochemical deposition (ECD) of copper. Copper ECD on the seed layer produces void-free fills in high aspect ratio features and is a process methodology of choice for metallization of the semiconductor substrate.
ECD of copper as conventionally carried out depends on use of organic additives in the plating solution of the bath in which the deposition is carried out. The bath also contains inorganic additives, and the ECD process is sensitive to concentration of both organic and inorganic components, since these components can vary considerably as they are consumed during the life of the bath. Only by real-time monitoring and replenishment of all major bath components can the semiconductor manufacturing process be assured of optimal process efficiency and yield.
The inorganics in the copper ECD bath include copper, sulfuric acid and chloride species, which may be measured by potentiometric analysis. Organic additives are added to the ECD bath to control the uniformity of the film thickness across the wafer surface, and include suppressor, accelerator and leveler species. The concentration of organic additives may be measured by cyclic voltammetry or impedence methods, or by pulsed cyclic galvanostatic analysis (PCGA), which mimics the plating conditions occurring on the wafer surface. PCGA is frequently used, and employs a double pulse for nucleation and subsequent film growth on the electrode, in performing abbreviated electrolysis sequences and using analytical sensors to measure the ease of metal deposition. Through chemical masking and monitoring of the plating potential, additive concentrations are readily determined.
A chemical analysis system that is advantageously employed for monitoring of copper ECD processes, utilizing potentiometric analysis for monitoring of inorganic components of the ECD bath, and PCGA analysis for monitoring of organic components, is commercially available from ATMI, Inc. (Danbury, Conn., USA) under the trademark CuChem. Such system utilizes a working electrode, typically formed of platinum, on which copper is cyclically plated, in a process sequence of cleaning, equilibration, plating and stripping.
In the operation of the CuChem™ system, it has been observed that in some instances the stripping operation tends to drive the platinum electrode to an excessively anodic condition. This anodic condition in turn results in changes in the state of the working electrode and loss of performance over time. Although such excessively anodic condition can be ameliorated to some extent by reducing the duration of the working electrode stripping step, and/or reducing current density in the working electrode, there is a need for a monitoring system that is free of such excessive anodicity in the working electrode.
Considering the electrochemical cell of the ECD monitoring system in further detail, with respect to potentiostatic measurements, in which a controlled voltage is imposed on the cell and resulting current is measured, and galvanostatic measurements, in which a controlled current is induced on the cell and the resulting voltage is measured, various configurations of analog circuitry have been successfully employed in monitoring systems.
In the application of such analog circuitry, measurement of both potentiostatic currents and galvanostatic voltages requires analog circuits including analog or mechanical switches, numerous operational amplifiers and even more numerous resistors. Each of such components of the analog circuit is both an error source and a noise source. The error/noise issues associated with such analog circuit componentry become even more significant in the nanoamp (10−9 amp) signal region that is desired for next generation ECD monitoring systems. Additionally, the desired use of a unipolar power supply in such nanoamp regime requires even more complexity of the analog circuitry, which also further increases the error and noise levels in the electrochemical process monitoring circuitry.
It therefore is desirable to provide ECD monitoring circuitry that affords a solution to such issues of circuit complexity, noise, accuracy, dual mode (potentiostatic mode and galvanostatic mode) operation, and unipolar power supply usage.